The present invention relates to a process for brominating fluorine- and chlorine-containing halogenated hydrocarbons by exchanging a chlorine therein with bromine by reaction with hydrogen bromide in the presence of a catalyst.
Catalysts for catalyzing the exchange of chlorine with bromine in chlorinated hydrocarbons which do not contain any fluorine are known in the art (see, e.g., Houben-Weyl, Methoden der Organischen Chemie, 4th Ed., vol. V/4, p. 356, or Z. E. Jolles in "Bromine and Its Compounds," publisher Ernest Benn Ltd., London, 1966, p. 384).
Yet, for reacting fluorine- and chlorine-containing hydrocarbons wherein the reactivity of the chlorine is particularly low, only the process which is disclosed in U.S. Pat. No. 2,729,687 is known up until now. According to this process, the substitution of chlorine by bromine is effected at a temperature of between 500.degree. and about 650.degree. C.
Disadvantages of this process are the high reaction temperature and, connected therewith, the very limited choice of reactor materials, as well as low degree of substitution and a low selectivity with regard to substituting only one of several chlorine atoms within a halogenated fluorohydrocarbon containing more than one chlorine atom per molecule. For example, when dichloro-difluoromethane is reacted with hydrogen bromide at a molar ration HBr:CCl.sub.2 F.sub.2 of 0.8 at a temperature of 600.degree. C. and a contact period of 22 seconds, only 5 mole percent are converted into bromo-chloro-difluoromethane and another 5 mole percent into dibromo-difluoromethane.